1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus such as a photocopier, a printer, and a facsimile machine, and also relates to a gear mechanism used therefor.
2. Description of the Related Art
A conventional electro-photographic image forming apparatus forms an image on a printer sheet in such a manner as to form a toner image by developing an electrostatic image formed on an image supporter, transfer this toner image to a printer sheet that is carried by carrying means, and fix the toner image on the printer sheet. As the image supporter, a cylindrical photoreceptor drum whose outer cylindrical surface is covered with a photosensitive coating film is typically used.
Since the photoreceptor drum must be regularly replaced for the sake of good image formation, the photoreceptor drum has a unit construction that allows the drum to be easily detached from the main body of the image forming apparatus (cf. Japanese Laid-Open Patent Application No. 2002-278364; published on Sep. 27, 2002). On this account, the photosensitive drum has driving force transmitting means by which the photoreceptor drum receives the rotative driving force from a rotative driving section of the main body of the image forming apparatus so as to rotate.
As this driving force transmitting means, gears, coupling means in which a concave-shaped driving section engages with a convex-shaped driven section, and the like are typically used.
When gears are adopted as the driving force transmitting means, it is necessary to keep the photoreceptor drum to smoothly rotate on account of good engagement of the gears, in order to suitably form an image by the photoreceptor drum. To achieve this, Japanese Laid-Open Patent Application No. 2000-147948 (published on May 26, 2000), Japanese Laid-Open Patent Application No. 2-262668/1990 (published on Oct. 25, 1990), Japanese Laid-Open Patent Application No. 2002-304030 (published on Oct. 18, 2002), and Japanese Laid-Open Patent Application No. 2000-267374 (published on Sep. 29, 2000) propose respective methods.
However, the tooth surfaces of a driving gear and a driven gear wear away in the course of the rotation of these gears engaging with each other, causing the engagement of these gears to go wrong even if these gears are at first engaged with each other without gaps or with suitable gaps. This brings about irregularity in the rotation of the photoreceptor drum, and the quality of a formed image may deteriorate. Taking this problem into account, the formation of gaps between the driving and driven gears is restrained even if the tooth surfaces thereof wear away to some extent, by utilizing the elasticity of the rotating shafts of the gears by narrowing the distance between the rotating shafts.
As shown in FIG. 11, a part of a driven gear 110 is inserted into the inside of a photoreceptor drum 100, and the driven gear 110 rotates around a rotating shaft 120. The driven gear 110 is driven by a driving gear 200 that engages with the driven gear 110 and rotates around a rotating shaft 210. The photosensitive drum 100 is pushed towards the rotating shaft 120, by a cleaning blade 300 that gathers toner remaining on the surface of the photoreceptor drum 100.
When the photoreceptor drum 100 and the members around the drum 100 are arranged as shown in FIG. 11, the distance between the rotating shafts 120 and 210 of the driving gear 200 and the driven gear 110 can be narrowed by, for instance, causing the rotating shaft 210 to tilt toward the rotating shaft 120, as shown in FIG. 12. It is noted that members such as the photoreceptor drum 100 are omitted from FIG. 12 for the sake of simplicity.
However, when the rotating shaft of one gear tilts toward the rotating shaft of the other gear as above, the driving gear and the driven gear may severely collide with each other if the photoreceptor drum is carelessly attached to the main body of the image forming apparatus. Such a collision damages and/or deforms the tooth surfaces, so that the state of engagement of the gears deteriorates and irregularity of the rotation of the photoreceptor drum is further facilitated.
That is to say, the photoreceptor drum 100 and the driven gear 110, which have not yet been fit with the main body of the image forming apparatus, are disposed as shown in FIG. 13. The photoreceptor drum 100 is then moved in the direction indicated by an arrow A in FIG. 14, so that the driven gear 110 is gradually fit with the rotating shaft 120.
In the present case, as described above, the rotating shaft 210 tilts toward the rotating shaft 120. That is, as shown in FIG. 12, these two rotating shafts are arranged in such a manner that, in the vertical direction, the distance (a) between the rotating shafts around the bases thereof is longer than the distance (b) between the rotating shafts around the tips thereof (b<a). Thus, as shown in FIG. 14, when the driven gear 110 is about to engage with the driving gear 200, the teeth of these gears 110 and 200 are likely to collide with each other as shown in FIG. 16, rather than properly engaging with each other as shown in FIG. 15. In this manner, the teeth of the gears collide with each other so that the tooth surfaces are damaged and/or deformed as described above.